摘要:

We propose and demonstrate a technique for artifically bunching the atomic steps on a vicinal substrate to form supersteps of almost arbitrary height. The process involves the etching of a grating of parallel grooves on the surface of a vicinal substrate followed by epitaxial growth that fills the grooves. Steps of height proportional to the period of the grating and the substrate misorientation angle are formed. The technique is demonstrated on a macroscopic scale for the InP/InGaAs material system using chloride transport vapor levitation epitaxy, resulting in InGaAs wire‐like structures confined both horizontally and vertically by InP. The growth of quantum wires and laterally periodic superlattices should be possible using this technique with proper scaling of parameters.

ISSN:0003-6951    

DOI:10.1063/1.101856

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Low‐temperature photoluminescence (PL) measurements of GaAs, AlxGa1−xAs, and AlAs samples grown by molecular beam epitaxy have been carried out to study the effects of hydrogen diffusion. Following exposure to a hydrogen plasma, the PL spectra of AlxGa1−xAs change. In particular, direct gap AlxGa1−xAs shows a strong increase in the total PL intensity whereas the PL spectra of indirect gap AlxGa1−xAs show an increase in the excitonic‐related recombinations after hydrogenation; the binary compounds present less dramatic changes. We interpret our results in terms of hydrogen passivation of deep and shallow centers (DX), whose densities are higher for aluminum concentration near the direct to the indirect gap crossover.

ISSN:0003-6951    

DOI:10.1063/1.101857

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A device physics model is developed to analyze spontaneous neuron‐like spike train generation in current driven siliconp+‐n‐n+devices in cryogenic environments. The model is shown to explain the very high dynamic range (107) current‐to‐frequency conversion and experimental features of the spike train frequency as a function of input current. The devices are interesting components for implementation of parallel asynchronous processing adjacent to cryogenically cooled focal planes because of their extremely low current and power requirements, their electronic simplicity, and their pulse coding capability, and could be used to form the hardware basis for neural networks which employ biologically plausible means of information coding.

ISSN:0003-6951    

DOI:10.1063/1.101858

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Photoelectric emission induced by a focused UV laser beam (&lgr;=257 nm) has been used to probe semiconductor surfaces. It was possible to distinguish between regions of different doping levels on a silicon surface. The spatial resolution was found to be limited only by the laser beam spot size.

ISSN:0003-6951    

DOI:10.1063/1.101859

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Theoretical predictions are presented of electronic and optical properties of the Si/Ge (2:6) superlattice grown on (001) SiGe buffers. It is shown that the buffer layer on which the superlattice is grown controls the polarization isotropy of the fundamental cross‐gap transition and may be used to engineer an optical window for direct gap behavior.

ISSN:0003-6951    

DOI:10.1063/1.101860

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

High voltage electron microscopy showed that a [001] twist boundary is abundant in the Bi‐Sr‐Ca‐Cu‐O superconductor when it is prepared by a solid‐state reaction method. It is certain that coincidence orientation boundaries such as ∑=13 and ∑=17 are preferred.

ISSN:0003-6951    

DOI:10.1063/1.102427

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We have fabricated submicron injection‐controlled NbN links with gain and measured their injection‐triggered superconducting‐to‐normal transition to occur in 0.5–2.6 ns at 4 K, governed by condensate dynamics. The transient pulse shape displays a rising‐edge kinetic‐inductance spike and a strong dependence on the amplitude of the injected quasiparticle current similar to microbridges driven by supercritical currents. A modified dynamic effective‐temperature model is used to interpret the transient time and the pulse shape.

ISSN:0003-6951    

DOI:10.1063/1.101861

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Coatings of YBa2Cu3O7−ywere deposited by electrophoresis on Cu, Ag, polycrystalline Al2O3, and single‐crystal MgO and YSZ substrates. Acetone was used as the electrophoretic vehicle and a field of 500 V/cm was applied in the deposition process. The deposited coatings were fired in air at or above 900 °C. For coatings on Ag, MgO(Ag), and YSZ(Ag) substrates, sharp superconducting transitions at ∼90 K were obtained. Intermediate layers were formed on Cu, Al2O3(Al), and YSZ(Ag) substrates as a result of interfacial reactions during firing. The interfacial reactions affected the superconducting transition and also led to the development of a preferred orientation in the coating.

ISSN:0003-6951    

DOI:10.1063/1.102428

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7material by physical sputtering. The technique can produce structures down to 0.5 &mgr;m or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20‐&mgr;m‐wide and 50‐&mgr;m‐long strips, each wired for four‐terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak‐link junctions with roughly 0.3 &mgr;m separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer‐sized superconducting structures.

ISSN:0003-6951    

DOI:10.1063/1.102429

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Bi(‐Pb)‐Sr‐Ca‐Cu‐O thin films were prepared on MgO(100) single‐crystal substrates by electron cyclotron resonance (ECR) plasma sputtering at substrate temperatures from room temperature to 590 °C. Pb‐doped superconducting as‐grown films were obtained above 560 °C. TheTcvalues of the Pb‐doped films prepared at 570–590 °C were 58–64 K which increased with increasing substrate temperature. Pb‐undoped as‐grown films obtained at 590 °C showed superconduction (Tc=30 K), but the films obtained below 580 °C were semiconductors. The grain sizes and contents of the 37 A˚ phase (110 K phase) were increased by the Pb doping into the as‐grown Bi‐Sr‐Ca‐Cu‐O films.

ISSN:0003-6951    

DOI:10.1063/1.101862

出版商:AIP    

年代:1989

数据来源: AIP